The present invention relates in general to a method of surface treating a water-absorbent composition of matter to improve its water dispersibility and to reduce its lumping tendencies, and the dry water-absorbent compositions produced thereby.
Absorbent compositions of matter based on anionic poly-electrolytes generally fall into three classes. The first class consists of the substantially water-soluble hydrocolloid materials long known in the art. Such materials merely increase the viscosity of (i.e., thicken) the liquid exposed thereto; in the presence of an added liquid excess, such materials lose their power to retain the viscosity they previously achieved. Nonetheless, such water-soluble materials are useful in a number of applications where they will be exposed to only a controlled amount of fluid; for example, as adhesives, thickeners, coatings, textile sizing agents, water treatment and purification agents, drainage aids, etc.
More recently, there has been a high degree of activity in the area of water-insoluble particulate hydrocolloid absorbent compositions of matter and products using the same, such as absorbent dressings, diapers, catamenial tampons, and the like for absorbing aqueous fluids such as water, urine and other aqueous body exudates. Such substantially water-insoluble compounds maintain their particular character as they imbibe and absorb many times their weight of surrounding liquid, and in doing so, swell. The compounds are capable of absorbing at least 15 times their weight of fluid. In doing so, each individual absorbent particle swells or enlarges several hundred percent times its individual parameter without destruction of its initial particulate integrity. Each particle maintains the approximate shape and geometry it had before contact with liquid, but the dimensions are greatly enlarged to provide for the binding of the liquid absorbed therein. As the water-insoluble compound accepts liquid, it substantially immobilizes the liquid therein, and the resulting particulate liquid-swollen structure is gelatinous. The mass of swollen particulate water-insoluble particles defines an aciniform structure since each individual absorbent particle is a greatly enlarged particle, having become liquid-swollen or grape-like or acinus in form due to the liquid it has absorbed. The individual swollen particles are tacky and hence form a clustered mass of liquid-swollen particles. The particles remain in an acinus form state despite the presence of liquid in excess of their ability to absorb. The liquid-swollen particles bind their absorbed water tightly, but upon drying, they become dehydrated and return more or less to their original size. At this time they can operate more or less as before to absorb and bind liquids.
The water-insoluble absorbent compositions described above are generally formed in either of two ways (corresponding to the second and third classes). As described in U.S. Pat. NOs. 3,628,534; 3,669,103; and 3,670,731; one or more monomers, which if homopolymerized would form a water-soluble polymer, are copolymerized with a monomer which covalently crosslinks the molecule and introduces a limited water-insolubility. In general, the degree of crosslinking is contained so that the polymer network of the hydrocolloid is not soluble in aqueous media, yet remains flexible and swells as the aqueous media is absorbed within its structure.
Alternately, as described in the U.S. patent application Ser. No. 556,291, filed Mar. 7, 1975, such water-insoluble compositions may be formed through the polymerization and ionic complexing of one or more monomers, which if homopolymerized would form a water-soluble polymer, with polyvalent metal cations having a valance of at least three. The advantage of the ionic complex compositions over the covalently crosslinked compositions is that the former are easy to shape and apply to substrates for particular applications since they may be uncomplexed at an elevated pH and re-complexed at a lower pH.
Regardless of the relative merits and demerits of the three classes of water-absorbent compositions of matter, all are subject to the same problem variously termed in the art as lumping, poor "wet-out" or poor water dispersibility.
Upon exposure to the aqueous fluid to be absorbed, the absorbent compositions exhibit poor dispersibility in the aqueous medium and at least initially, form visible clumps consisting of fluid-swollen material on the surface and relatively dry material on the inside. Expecially when used in the form of relatively fine powders, the exposed surface of the absorbent composition forms a gel-like structure which inhibits the passage of the aqueous fluid therethrough. Accordingly, the absorbent capacity of the compositions is at least initially reflective of only the absorbent capacity of the surface, and not the absorbent capacity of the interior as well. A slow rate of exposure of the absorbent composition to the aqueous medium to be absorbed, the use of relatively large granules of the absorbent composition, and agitation of the aqueous medium during exposure tend to reduce the occurrence of lumping, while agitation of the aqueous medium after exposure and the passage of time tend to dissolve lumping once it has occurred. Nonetheless, in many instances the specific application in which the absorbent composition is used precludes resort to one or more of the aforementioned expediencies, and the need remains for an absorbent composition having improved aqueous dispersibility (i.e., better wet-out and less lumping).
One object of the present invention is to provide a process for improving the aqueous dispersibility or wet-out of an absorbent composition based on an anionic poly-electrolyte.
Another object is to provide such a process which is effective with water-soluble compositions, covalently crosslinked water-insoluble compositions and ionically complexed water-insoluble compositions.
A further object to provide such a process which reduces the amount of lumping exhibited by the absorbent composition upon rapid exposure thereof to large quantities of the aqueous fluid to be absorbed.
Stll another object is to provide such a process which enables the absorbent composition to reach full dispersion in the aqueous fluid to be absorbed in a reduced period of time.
A final object is to provide an absorbent composition treated according to such a process.